Capacitively coupled cavity resonator



Sept. 27, 1960 M. MULLER CAPACITIVELY COUPLED CAVITY RESONATOR FiledDec. 3, 1957 INVENTOR M- M'Liuer yaw.

ATTORNEY Unite StatesParent 2,954,536 CAPA'CITIVE'LY COUPLED CAVITYRESONATOR Martin Miiller, Pforzheim, Germany, assjignor .toInternational Standard Electric, Corporation, New York, N .Y., acorporation of Delaware" I Filed Dec. 3, 1957, so. No. 700,498 Claimspriority, application Germany "Dec. 6, 1956 4 Claims. to]. sea-73resonator circuits or their combinations forming fre- .quency filters,can be built up out of cavity resonators.

By suitably selecting the dimensions, the tuning or .adaptation and thecoupling elements of such resonators among each other and with theconnecting transmission line it is possible to obtain similar propertiesas with the filters composed of concentrated elements.

It is possible to realize four diiferent kinds of coupling betweennonresonant and resonant line sect-ions, i.e. the capacitive (C) orinductive (L) coupling either with the resonator current (H) or with theresonator voltage Relatively simple to realize are the combinations CEand LH, which are generally employed. However, they bear thedisadvantage of getting a broader band- Width at the tuning through withan increasing frequency and, in this way, result in unsymmetricalamplitude and delay characteristics versus frequency. In additionthereto, as is well-known, all line resonators have unwanted pass bandsapproximately at all integer multiples of the fundamental pass band. Thecombinations CE and LH become of a broader bandwidth as the octavesincrease, so that they have a very poor harmonic suppression. It isregarded as a further disadvantage of these combinations that theirequivalent circuit is arranged inside the resonator, which has anunfavorable effect with respect to branching circuits or when such afilter is connected to transmission lines of dilferent characteristicimpedance at input and output.

The remaining two combinations CH and LE do not have thesedisadvantages. With resonators of a smaller relative bandwidth, however,the LE type combination is practically difiicult to realize, becauselarge series inductances cannot be unambiguously represented with or inthe case of microwaves. Also the CH type combination is encountered bysimilar ditficulties, because diaphragm couplings of the conventionaltype with a sufiiciently high susceptance are incapable of beinggoverned with regard to the precision required. The application of theembodiment of a coupling capacity, which is customary with coaxial lineresonators, by means of thick screw-stamps, in cavity resonators bear-sa further disadvantage, namely the excitation of higher H waveforms andtheir disturbing consequence caused by the relatively long decaydistance.

The invention, therefore, is aimed at reaching the advantages of acapacitive H-field coupling of cavity resonators preferably withrectangular waveguides by avoiding the aforementioned disadvantages.

In this respect it is proposed, in accordance with the invention, thatthe inside space of preferably a rectangular waveguide, that isconnected to the cavity resonator, is terminated by a capacity which isarranged between the two broad side wall of the said waveguide, so thatthe spacial dimensions of this capacity substan- 2,954,536 PatentedSept. 27, 1960 tially extend equally over the entire width of the wave,guide, an .so tha t e adjo ng ty r s n o i o tiled via t is c pa i y byy of a c rr n o H-cou: a as- An advan eous further e b e of t int/en: tn c ists n th a a coupli c pa i y i the i s d space of the wayeguidethere serves a conductive rod arranged parallel with the broader sideWalls a there.- pf, and that between these side walls and thevconductive rod there is respectively formed a slot-like coupling gap ofthe length a.

This can be accomplished by employing rectangular av y esonato s o asame Width a nd a same e ;b as the connecting or adjoining waveguide. Itis par.- ticularly appropriate that its height [2 is greater than that.of the coupled flat type rectangular waveguide, and that the resonatoris excited in the H -mode.

One possibile form of a resonator, in which the width a .of theconnecting waveguide is maintained, is the regular cylindrical E-resonator of the same cylinder height, which is being excited from agenerating line.

Finally there has proved-to be particularly favourable a modification ofthe two aforementioned types of resonators in such a way that the cavityhas a cylindrical shape withan oval cross-section, that the rectangularwaveguide is coupled along a generating line at the narrow-side of thecylinder, and that the resonator is ex cited in a degenerated H 1circularor H rectangular- .mode.

In order to achieve a particularly high coupling sus- .eeptance at whichnodisturbing H -wave forms are excited, ;there is proposed a couplingwhich is so ,dirnensioned that the conductive rod or bar has a circularcross-section and the inside space of the waveguide at the point of thecoupling, is in such a way extended or enlarged by a correspondingboring in parallel with the side walls a, that the coupling gaps, whichare formed by the conductive rod, have a segment-shaped crosssection.

A tunable coupling can be accomplished in that the circularcross-section of the conductive rod is flattened on one or two sides, sothat the coupling gaps will have a width that may be varied by turningthe cylinder.

It is another advantage of the invention that the coupling rod can beinserted from the outside. The bandwidth and the transmission ratio ofthe resonator can thus also be varied within wide limits .in that thecoupling rod is capable of being interchanged at will.

Capacitively coupled cavity resonators according to the invention areespecially suited to the purpose of manufacturing frequency filters inthat either corresponding previously balanced resonators, by theinterconnection of waveguide pieces, are connected to form alambdaquarter-coupled filter, or that cavity resonators, directlyadjoining the coupling capacity, are combined to form 9. directlycoupled multi-circuit filter.

In the following the invention will be described in particular withreference to some examples of embodiment shown in Figs. 1, 1a, and 2 ofthe accompany drawmg:

Fig. 1, in a longitudinal section, shows a cavity resonator 1 with atuning stamp 2 and the connected rectangular waveguides 3 and 4. In thisexample the cavity is designed as an oval H -resonator. This shape hasproved to be particularly flavourable for both the manufacture of thecasting and the necessary surface processing.

The reference numerals 5 and 6 in the drawing denote the coupling rods,which according to the invention, are arranged freely in respectivelyeach a boring that somewhat expands the waveguide. The rod or bar 6 issome.- what flattened on one side, so that, by turning it within theboring, the width of the gap can be varied at least on-one side withincertain limits.

With a resonator of the shape or type as shown in Fig. 1 of the drawingsthere can be obtained, within the 4000 mc. range, bandwidths between and100 me. and more at only the slightest natural attenuation, permittingalready without changing the position of the coupling rods to tune theresonance frequency with an almost constant bandwidth throughout a rangeof 600 me. and even more. When flanging several such resonators with acorresponding distance (about one quarter of a wavelength between thecoupling cylinders) behind each other then it will be possible to set upfrom individual or single resonators, which are tuned correspondingly inthe most simple manner, a multi-circuit filter possessing propertiesthat can be varied at will within wide Figure la illustrates a sectionalview taken along lines A-A of Fig. 1. This figure illustrates theposition of coupling rods 6 in rectangular waveguide 4. Letters a and brepresent the width and height respectively of waveguide 4.

In a similar way there may be built up a multi-cavity filter from anumber of resonators, which are directly coupled with one another, as isshown in Fig. 2 in the form of a three-section filter, wherein 1 denotesthe three cavities of the same type, which are designed in thisexample-as H -rectangular resonators. Reference numeral 2 indicates thetuning screws (also termed screw stamps or tuning stamps hereinbefore),3 the coupling cylinders (or rods) which, if so required, may havedifferent diameters, and 4 the connecting waveguides.

Also a filter of this type is widely tunable without any noteworthychange of the bandwidth and of the transmission properties. It furtherbears the added advantage of having a substantiallyfrequency-independent length, because between the individual cavityresonators there are arranged no line sections or pieces, the length ofwhich would have to be tuned or matched.

While I have described above the principles of my invention inconnection with specific apparatus, it is to be clearly understood thatthis description is made only by way of example and not as a limitationto the scope of my invention as set forth in the objects thereof and inthe accompanying claims.

. What is claimedis: I

1. An arrangement 'for coupling the rectangular waveguide, having wallsof major and minor dimensions to a cavity resonator, having aperturestherein substantially equal to'the major dimension of said waveguidecomprising means vforconnecting the walls of said waveguide to saidresonator about said aperture, and a rod extending across the entirewidth of said waveguide across its major dimension adjacent saidaperture.

2. An arrangement according to claim 1, wherein said means comprisesextensions of said waveguide walls of said major dimensions enlargedwith respect to the normal wall, said rod being mounted in said enlargedregion.

3. An arrangement as in claim 1, wherein said rod has a non-symmetricalcross-section.

4. An arrangement as in claim 1, wherein said rod has a circularcross-section having one of its sides flattened so that the capacity ofthe coupling between said resonator and said waveguide can be varied byturning the rod.

References Cited in the file of this patent UNITED STATES PATENTS

